The present invention relates to supplying a compensating leading or lagging reactive current to an A-C electric power system for stabilization of voltage, and more particularly, to a thyristor switched inductor circuit that is operated both during normal and transient conditions to develop such compensating currents.
It is known that electric power systems which supply highly erratic reactive loads, e.g., electric arc furnaces, are typically characterized by poor voltage regulation. Consequently, these power systems often exhibit an undesirable flicker. Compensating and regulating systems, herein simply referred to as regulating systems, suggested to alleviate this condition are disclosed in several of my issued U.S. Patents including: U.S. Pat. Nos. 3,143,315; 3,936,726; 3,936,727 and 4,121,150. The foregoing patents are hereby incorporated by reference into the present application.
Compensating systems having a thyristor circuit formed from a parallel arranged thyristors and inductors providing a leading or a lagging reactive current for regulating the voltage of an A-C electric power system are well known. One such thyristor circuit is described in IEEE Papers F78-719-7 and F78-726-2 by Hammad et al, entitled "A New Generalized Concept for a Design of Thyristor Phase-Controlled VAr Compensators", Parts I and II, presented at IEEE PES Summer Meetings in Los Angles, Cal., July 16-21, 1978. The thyristor circuit of Hammad et al, is operated to supply a variable inductance for regulating voltage of an A-C electric system. During overvoltage stresses an emergency firing mode is provided mainly to protect the thyristors from the overvoltage stresses. The emergency firing mode places the thyristors into their fully conductive mode during overvoltage stresses, which, in turn, effectively places a fixed inductance value across the A-C electric system during the overvoltage stress conditions. The maintenance of such a fixed inductance is inconsistent with the principle of varying the inductance to compensate or smooth the A-C line voltages during the transient condition. It is desirable that regulation be provided by a thyristor circuit such as to smooth these overvoltage transient conditions.
Accordingly, a general object of this invention is to provide a thyristor-switched inductor circuit that is operated to provide a variable inductance so as to develop compensating currents both during normal and transient conditions.
Another object of this invention is to provide such a thyristor-switched inductor circuit which operates to provide increased inductive compensation during the transient conditions as compared to the inductive compensation provided during normal conditions.
These and other objects of the present invention will become apparent to those skilled in the art upon consideration of the following description of the invention.